Kernel: Stack pointer is validated when updated

This allows us not to fail stack pointer when in syscall since
interrupts use their own stack

kernel/userspace/userspace.cpp

@@ -8,5 +8,13 @@
 USERSPACE void userspace_entry()
 {
 	Kernel::syscall(SYS_WRITE, STDOUT_FILENO, "Hello World!", 12);
+
+	char buffer[128];
+	while (true)
+	{
+		long n_read = Kernel::syscall(SYS_READ, STDIN_FILENO, buffer, sizeof(buffer));
+		Kernel::syscall(SYS_WRITE, STDOUT_FILENO, buffer, n_read);
+	}
+
 	Kernel::syscall(SYS_EXIT);
 }